Problem: The scale on a map is 3cm : 2km. If the distance between two cities is 18km, how far apart in cm are the two cities on the map?
Solution: The scale means that every 3cm on the map corresponds to an actual distance of 2km. An actual distance of 18km is the same as 9 $\cdot$ 2km. The distance between the two cities on the map is 9 $\cdot$ 3cm, or 27cm.